Add text clarifying what an "unconnected node" is.

[gkellogg]

Fixes #418.

[gkellogg]

@dlongley If this resolution is satisfactory, we can just merge it.

[dlongley]

typo: "effectivly" => "effectively"

[dlongley]

I can see someone (who does not read the rest of the spec, which is common) getting confused by this wording and also thinking this is illegal:

{
  "foo:bar": {
    "@id": "http://example.org/1"
  }
}

As that nested node contains only an @id. Perhaps a slight tweak:

This effectively prohibits unnested, empty node objects and unnested node objects that contain only an <code>@id</code>.